1. Field of the Invention
The present invention relates to a suspension coil spring used in a strut suspension for an automobile.
2. Description of the Related Art
Recently, strut suspensions are widely provided as suspensions for automobiles. The strut suspensions are configured such that a shock absorber, including a cylinder and a rod that is slidably supported by the cylinder, is used as a strut for determining a position with respect to a wheel, and a suspension coil spring is provided at outer peripheries of the rod and the cylinder.
In the above described structure, an upper end of the rod is connected to an automobile body via a strut mount or the like, and a lower end of the cylinder is rigidly connected to a knuckle that rotatably supports the wheel. The knuckle is pivotally connected to the automobile body via a lower control arm. The suspension coil spring is provided between an upper seat at which an upper end of the rod is fixed, and a lower seat at which a lower end of the cylinder is fixed in a compressed state.
Such a strut suspension has merits such as the number of components is small, the structure is simple and a space necessary to be installed is small, compared with other independent suspensions.
However, a bending moment is generated in a strut suspension due to a shift between a strut axis and a load input axis (an axis connecting a tire contacting point and an upper mount point of a strut). The bending moment causes the generation of a lateral force that is different from a sliding direction of a shock absorber, which increases friction of a rod to prevent a smooth operation of the shock absorber and which becomes a factor to worsen ride quality of an automobile.
Various suspension coil springs are offered in order to reduce such a bending moment. For example, Patent Document 1 offers a structure in which a suspension coil spring is decentered with respect to a strut, and including a pigtail end turn that is connected after being decentered. Patent Document 2 offers a suspension coil spring configured such that a center line of the spring has an S shape under an unloaded state.
Further, Patent Document 3 offers a suspension coil spring in which a plurality of protrusions are provided at its end turn portion, and configured such that the protrusions selectively contact a spring seat in accordance with the magnitude of an applied load. Further, Patent Document 4 offers a suspension coil spring in which strong striking portions are provided at an upper end turn and a lower end turn, respectively.